The invention relates to acoustic loudspeakers.
To provide the greatest listening pleasure, an acoustic loudspeaker system should strive to meet several basic requirements. First, it must be capable of reproducing very low frequencies, typically below 30 Hz, that are felt and not heard. Second, it must be capable of reproducing overtones of high musical notes. Third, it should have a relatively flat frequency and phase response over the full range of human audible frequencies, from about 20 Hz to about 20,000 Hz in order to reproduce sound with fidelity to the source. Fourth, also to be faithful to the source, the system should recreate whatever spatial illusions are contained in the source material. For example, most music sources are encoded for stereo reproduction using two channels. Two, spatially separated and phase-synchronous infinitesimal point sources of acoustic energy theoretically provide the best stereo imaging. These types of sources are able to create the illusion of sound originating from any point along a line extending through both point sources. Therefore, a loudspeaker system for stereo encoded audio sources should imitate as closely as possible two infinitesimally small point sources of acoustic energy. Fifth, to accommodate wide dynamic ranges, a loudspeaker system must be able to handle signals with power sufficient to reproduce low frequencies at loud volumes without distortion to the sound or damage to the speaker.
Conventional belief is that a single acoustic driver cannot deliver a frequency range and power handling capability required for high fidelity sound reproduction demanded by audiophiles. Characteristics of a transducer that optimize it for high frequency sound reproduction are often opposite of those that are optimum for a driver for low frequency reproduction. Therefore, most loudspeaker systems rely on two or more acoustic transducers or drivers per channel. Each driver of a channel is responsible for reproducing sounds in only in certain portions of the audible range. By utilizing multiple drivers per channel, each driver may be optimized to operate within a selected portion of the acoustic range. An electrical circuit, known as a cross-over network, splits portions of the energy of the input signal between the drivers based on its frequency and feeds it to the different driver.
Despite their widespread acceptance, multi-driver speakers have several drawbacks. First, cross-over networks distort the electrical sound signal, thus introducing distortion into the sound reproduced by the loudspeaker system. For example, cross-over networks naturally cause phase distortion in incoming signals: higher frequencies will be phase shifted with respect to the lower frequencies. Phase shifting results in a loss of imaging information, causing the music to sound xe2x80x9cmuddy.xe2x80x9d Cross-over networks therefore sometimes employ circuits to correct phase distortion. These cross-over networks will often introduce other types of distortion and possess non-linear responses. Second, multi-driver speaker systems tend to be larger and have more components, thus making them more expensive, bulkier and less mobile. Third, a multi-driver speaker does not satisfactorily represent a point source of acoustic radiation for a single channel, as a channel is obviously radiating from multiple points. Thus, they cannot achieve the best stereo imaging.
Despite the motivation for creating a broadband acoustic driver, the problems of using a single driver to reproduce at equal levels high notes with clarity and low notes with physical impact have been difficult to overcome.
A conventional acoustic transducer has a relatively stiff or rigid diaphragm which reciprocates along a linear axis. For reproducing low frequencies, the diaphragm has preferably a concave, cone shape. For high frequencies, it may be flat or convex. To vibrate the diaphragm, an electrical signal representing the sound wave to be reproduced flows through a coil mechanically connected to the diaphragm. The coil is situated within a fixed magnetic field, causing the coil to reciprocate with changes in the current. The coil is formed from one or more lengths of wire wrapped around a support structure. Typically, the edges of the diaphragm are attached to a basket shaped frame using a compliant, slightly resilient, material. The coil is centered within a gap referred to as a xe2x80x9cflux gap,xe2x80x9d formed between cylindrically shaped pole and a donut-shaped magnet assembly.
To provide the most accurate sound reproduction, the movement of the coil in response to the electrical signal and the coupling of the movement of the diaphragm to the air in response to the movement of the coil must be linear. Unfortunately, the responses of these elements to the sound signal are rarely totally linear, especially over the entire audible range. The diaphragm couples the mechanical energy of the moving coil to the air, thereby causing the air to vibrate and setting up acoustic waves. At lower frequencies, the diaphragm can be thought of as behaving like a simple mechanical piston pushing volumes of air. At low frequencies, a lot of power is required to push large volumes of air, particularly at loud volumes. Therefore, to sound low notes with great volume a speaker must be capable of handling a lot of power, particularly the mechanical stresses from the strong electromagnetic forces and resulting heat.
For good low frequency response, a driver is needed which is mechanically strong and powerful in order to move larger amounts of air. Thus, a stiffer diaphragm with a large surface area is preferred. However, a large, stiff diaphragm means more structure, and thus more mass. More mass means less efficiency, and thus more power to reproduce the same loudness. More power means that a more massive coil is required to handle the mechanical and thermal stresses resulting from the power. However, more mass in the moving parts inhibits the driver""s ability to reciprocate at higher frequencies. Also, it is more difficult to control coupling of the movement of the coil to the air through a large diaphragm and its natural resonances. A smaller diaphragm could be used to sound bass notes, but a longer throw or stroke of the coil would be required to move the same amount of air. However, a longer stroke necessitates either a magnetic field of greater magnitude or a longer coil in order to provide a sufficiently high electromotive force (EMF). Furthermore, a greater coil length means greater induction. Thus, the length of the coil is limited. A long stroke also requires the coil to move at a higher velocity. Higher velocities will create a higher back EMF, which resists travel of the coil and ultimately limits the ability of the driver to reproduce low frequencies.
At higher frequencies, the diaphragm behaves more like a radiating transmission line. The rapid vibrations of the coil cause not only linear movement of the diaphragm, but also mechanical vibrations in the diaphragm which radiate from the points where the coil is attached, outwardly to the edge of the diaphragm. Depending on the material, size of the diaphragm and how it is attached to the suspension, these vibrations may resonate at certain audible frequencies, thus adversely affecting the linearity of the coupling of the mechanical movement of the coil to the air. Although there may be mechanical deformation of the diaphragm at all frequencies, at high frequencies the effect of resonant vibrations will have a substantial impact on the sound, with certain frequencies being noticeably enhanced and others degraded. Reproducing a high frequency sound also requires the coil to be quickly accelerated. Thus, a near zero mass coil and diaphragm is theoretically ideal. Furthermore, a smaller diameter diaphragm is preferred. A larger diameter diaphragm tends to be more directional, exacerbating the directional nature of high frequencies.
Attempts have been made to accommodate the demands of high and low frequencies in a single, broad band acoustic driver, particularly in the area of reducing the mass of the moving parts of the driver. For example, as shown in U.S. Pat. Nos. 4,115,667 and 4,188,711 of Babb, the conventional rear suspension for the coil is replaced with a low friction bearing made of TEFLON(copyright). The bearing is formed at the bottom of the coil, opposite of where it connects to the diaphragm, and encircles and rides on the post. The coil remains centered within the gap without the extra mass of the rear suspension and its spring forces interfering with movement of the coil. The coil therefore can move more freely and accelerate faster, which aids in moving the coil long distances when using a longer throw coil to sound bass notes. A low friction bearing can also be added around the circumference of the top end of the post. Lightweight, stiff metal alloys have been used to form diaphragms. Coil forms (structures for supporting windings of coils) have been made from high strength, thermally resistant materials such as KAPTON(copyright). To provide a low mass, compliant suspension for the diaphragm, a stamped synthetic foam having a very low density with good dampening and resonance characteristics is used.
Nevertheless, although not recognized in the art, there still exist problems. One such problem comes from the fact that a coil undergoes great mechanical stress from the EMF generated by the magnet and the current running through the coil, as well as great thermal stress from the substantial heat generated when large currents flow through the coil during reproduction of loud notes. Despite the use of lightweight, stiff materials, a low mass coil capable of sounding both high and low frequencies will naturally tend to be weaker and thus more easily deformed by the mechanical and thermal stresses present during reproduction of high power sounds. A low mass coil also cannot store heat for later dissipation. Thus, during extended periods of loud notes, a low mass coil will tend to get very hot and possibly damaged. Furthermore, TEFLON(copyright) is not structurally strong and tends to shrink in heat, thus resulting in increased drag of the coil""s bearing on the post and deformation under high thermal and mechanical loads. A deformed coil cannot sound notes as accurately and will tend to rub against the walls defining the flux gap, causing noticeable distortion of low notes and extraneous noises at midrange frequencies.
When a full range driver is designed to have a flat frequency response over the entire audible range (20 Hz to 20,000 Hz) it must have a large enough diaphragm to displace enough air to produce the low frequency notes (20 Hz to 60 Hz) at adequate sound pressure levels. This minimum size places a heavy burden on achieving adequate performance in the high frequency range (5000 Hz to 20,000 Hz). If this driver is made with a metal cone, for optimum strength to mass properties, it tends to resonate or xe2x80x9cringxe2x80x9d at certain high frequencies. This resonance can be heard by, and is objectionable to, most audiophiles. As the size of the metal cone grows it becomes more difficult to control these resonance""s.
Another problem associated with this minimum diaphragm size is that, the larger the diaphragm, the more difficult it is to achieve a smooth angular dispersion pattern over the entire audible frequency range. An even dispersion pattern is required for a loudspeaker driver to function like an ideal point source driver, and to thus achieve a truly accurate audio image that extends beyond a narrow xe2x80x9csweet spotxe2x80x9d to cover the whole vertical and horizontal area in front of a pair of audio drivers.
One objective of the invention is to improve performance of an acoustic driver by overcoming one or more of the aforementioned problems. An example of loudspeaker employing the invention, in its preferred embodiment, is summarized below.
To overcome the problem of extraneous noises introduced by a voice coil caused by mechanical deformations in the voice coil and its misalignment with a cylindrical element on which it reciprocates, an acoustic driver of a loudspeaker is provided with a plurality of ridges that extend from an outer surface of the cylindrically-shaped element. (The cylindrical element may take the form of a solid or hollow pole, and may include a sleeve over the pole.) The ridges are linear and run generally in an axial direction, along a length of the pole where the voice coil reciprocates. Each ridge has a low friction surface. Instead of rubbing directly against the cylindrical element, the voice coil will rub against the ridges, thus reducing some of the noise that would otherwise occur due to rubbing. Additionally, each ridge may be made compressible to absorb some of the energy associated with the forces on the voice coil as it moves toward the pole.
In one disclosed embodiment of an acoustic driver employing this feature, each ridge is oriented in a helical fashion about the pole. With this arrangement, the flow of air along the pole that is caused by displacement of a voice coil within an air gap formed between the pole and a surrounding magnetic structure is not blocked, while providing greater chance that the coil contacts more than one ridge. The resiliency of the compressible ridges, and thus their energy absorbing effect, can be altered based on the internal structure of the ridge.
Another feature of the loudspeaker directed to overcoming the problem of extraneous noise is a voice coil that has a relatively stiff structure, created in part with a ceramic or epoxy material, that is coupled to a diaphragm, and a relatively flexible multiple layer structure at the terminating free end having dampening properties. The structure includes, in a preferred embodiment, two layers of Kapton(copyright) tape, each a flexible sheet of material possessing good tensile strength, between which is wound a portion of the coil. The layers of tape that extend beyond the rearward portion of the coil are held together by a tacky silicon adhesive to provide viscous dampening of the relative movement of the two layers.
Generally, it is preferred that a coil be stiff in order to provide a good coupling of its translational energy to the diaphragm. However, the coil will tend to resonate at frequencies determined in part by the stiffness of the coil. With a hard end and a soft end, an impedance mismatch is set up, dampening the resonance. Furthermore, a dampened flexible end of the coil acts as a non-reflective termination. This keeps the audio frequency energy that is generated by the coil from being reflected back from the end of the coil. The energy reflected be a hard, reflective boundary would be phase shifted and would cause peaks and valleys in the loudspeaker frequency response. Furthermore, when used in combination with compressible ridges on a pole that acting as bearings, two relatively soft and dampened structures will interact, further reducing the noise caused by rattling. The flexible structure also possesses, in a preferred embodiment, a thin tapered end for the coil that reduces the turbulence and air friction that results from the bottom end of the coil being pushed and pulled through the air. Less turbulence means less noise is generated, less friction means more efficient operation.